Os

XD

a

ARRAA

KOEFY

1

atdaPSuci

Oigacncioae

0d

Applied Catalysis A: General 381 (2010) 114–120

Contents lists available at ScienceDirect

Applied Catalysis A: General

journa l homepage: www.e lsev ier .com/ locate /apcata

xidative dehydrogenation of ethane with oxygen catalyzed by K–Y zeoliteupported first-row transition metals

ufeng Lin, Kenneth R. Poeppelmeier, Eric Weitz ∗

epartment of Chemistry and Institute for Catalysis in Energy Processes, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA

r t i c l e i n f o

rticle history:eceived 21 October 2009eceived in revised form 19 March 2010ccepted 24 March 2010vailable online 31 March 2010

a b s t r a c t

Fe, Co, Ni, and Cu oxide loaded K–Y zeolites were synthesized using a two-step method, in which transitionmetals in the zeolite framework reacted with the anions in solution to form an insoluble salt which, whencalcined, is expected to form an oxide that is anchored to the inner and/or outer zeolite surface(s). Thecatalytic properties of these zeolites, for the oxidative dehydrogenation of ethane (ODHE) to ethylenewere then investigated. An ethylene selectivity of close to 80%, at an ethane conversion of >20% was

eywords:xidative dehydrogenationthaneirst-row transition metalszeolite

achieved on a nickel oxide loaded K–Y catalyst. This is a higher ethylene selectivity, at higher ethaneconversion, than was achieved using a previously reported Ni/KY catalyst. The effects of both reactiontemperature and the O2/C2H6 ratio on the catalytic performance were examined. The apparent energy ofactivation for the ODHE reaction on the nickel oxide loaded K–Y catalyst was 51.5 ± 0.7 kJ/mol. The C2H4

selectivity at zero conversion (S0) on nickel oxide loaded K–Y was estimated for various O2/C2H6 ratiosres. T

ritica

at a number of temperatuto the relative values of c

. Introduction

Over 100 million metric tons of ethylene are produced annu-lly [1], with steam cracking of longer chain hydrocarbons beinghe primary method for ethylene production [2]. Oxidative dehy-rogenation (ODH) of ethane (ODHE) is potentially a lower energynd less capital intensive method for the production of ethylene.etroleum is currently the main feedstock for the steam cracking.ince petroleum is expected to be exhausted many years before nat-ral gas, a source of short chain alkanes, it is desirable to develophemical processes that employ small alkanes to yield more chem-cally useful olefins.

Although a large body of ODHE research has been reported, noDHE process is currently a clear candidate to replace steam crack-

ng [3]. This is primarily because catalysts are not available thative high conversions of alkanes along with high selectivity forlkenes. In the literature ODH catalysts can be divided into threeategories [4]: catalysts based on (i) reducible metal oxides, (ii)oble metal-based catalysts, primarily Pt-based catalysts, and (iii)atalysts that generate radicals to initiate homogenous reactions

n the gas phase (these typically contain alkali earth oxides andxides of rear earth metals). For the first type of catalysts, vanadiumnd molybdenum oxide-based catalysts have been studied mostxtensively [3,5–8]. In contrast, there has been less work with the

∗ Corresponding author.E-mail address: weitz@northwestern.edu (E. Weitz).

926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apcata.2010.03.049

he implications for the reaction mechanism of the S0 values, with regardl rate constants are discussed.

© 2010 Elsevier B.V. All rights reserved.

first-row group VIII metals for ODH catalysis [9–11]. However, sincethese metals are plentiful and relatively inexpensive they would behighly desirable as catalysts. Additionally, reports on ODHE reac-tions using zeolite supports, especially microporous zeolites, arerelatively sparse [12,13]. In previous work [13] we have shown thatNi loaded acid Y zeolites exhibited an ethylene productivity of upto 1.08 gC2H4

/(gcat h) with a selectivity of ∼75%. Extended X-rayabsorption fine structure (EXAFS) showed that the active Ni cata-lysts contain both Ni–Ni and Ni–O bonds. In the synthetic methodemployed in Ref. [13], Ni ions are initially incorporated into thezeolite framework and are subsequently reduced by H2 to neutralNi, which then anchors to the inner and/outer zeolite surface(s).

Zeolite supported transition metal compounds can be preparedin a variety of ways, and the method of preparation can greatlyaffect the catalytic property of a catalyst as a result of changes inparticle size and shape, and oxidation states, etc. For example, Sunand Sachtler [14] have reported three methods of preparation of Mnloaded MFI zeolites (wet-ion-exchange, chemical vapor depositionand solid state ion-exchange), where wet-ion-exchange leads tothe highest activity for NOx reduction with alkanes.

In this paper we explore a synthetic method for metal (M = Fe,Co, Ni, and Cu) loaded zeolites catalysts for ODHE reactions thatdiffers from what was employed in Ref. [13], and we investigate

the catalytic performance of these zeolites and the mechanisticimplications of changes in yields as a function of reaction param-eters. In the present study, as in Ref [13], the transition metal ionswere initially incorporated into the zeolite cationic sites by ion-exchange. They then reacted with the anions in solution (OH− and

is A: G

pwipi

otHtiCair

2

2

(iwnzsz1wtfadccbaca

2

zes(d0raecflaT

2

5aow

to those for the Na–Y starting material (i.e. they are all missing themetal or oxide phase contribution) [15].

Fig. 2 shows the H2-TPR profiles of the M/K–Y type II and M–Yzeolites. When the Ni and Co loaded zeolites are treated with K3PO4solution, H2-reduction occurs at a lower temperature for M/K–Y

Table 1Chemical composition of metal (M = Ni, Cu and Fe) loaded Y zeolite measured byICP-AES. The metal to Al ratios are calculated based on the molar concentration, i.e.the [M]/[Al] ratio in the solution for the dissolved samples.

Catalyst M/Al Na/Al K/Al P/Al

Ni/K–Y type II 0.36 0.06 0.93 0.01Co/K–Y type II 0.33 0.06 0.95 0.01Cu/K–Y type II 0.36 0.05 0.95 0.05Fe/K–Y type II 0.37 0.06 0.05 0.02Ni–Y 0.36 0.33

X. Lin et al. / Applied Catalys

ossibly a small amount of PO43−) to form an insoluble salt that,

hen calcined, is expected to form an oxide that is anchored to thenner and/or outer zeolite surface(s). All catalysts examined in theresent work employ a basic zeolitic support with the exchanged

on being K+.An ethylene selectivity of close to 80% at ethane conversions

f >20% was obtained on K–Y supported Ni catalyst. Compared tohe Ni/K–Y catalyst, where the Ni component was anchored using2-treatment [13] (13.7% conversion with 72.9% ethylene selec-

ivity was reported for 100 mg Ni/K–Y) the Ni catalyst reportedn this paper gives a higher selectivity for ethylene at a higher2H6 conversion. The effects of different reaction conditions arelso examined with regard to their mechanistic implications, andnformation is deduced regarding the relative magnitudes of criticalate constants.

. Experimental

.1. Catalyst preparation

Transition metal (M = Fe, Co, Ni, and Cu) oxide loaded Y zeolitesNa–Y, Aldrich, with Si/Al = 2.4) were prepared using ion-exchangen conjunction with subsequent wet impregnation. Ion-exchanges

ere performed by mixing 200 ml of 0.1 mol/L metal sulfate oritrate (FeSO4, Co(NO3)2, NiSO4 and CuSO4) solutions with 6 g Yeolite (Sodium-form, Si/Al = 2.4, Aldrich). The mixtures were thentirred for 6 h at 88 ◦C. After filtering, the transition metal-loadedeolites were washed with large amounts of water and dried at10 ◦C. The catalysts obtained are henceforth denoted as M–Y,here M = Fe, Co, Ni, and Cu. The resulting powders were added

o 100 ml of 0.8 mol K3PO4 (pH = 13), and the mixtures were stirredor 2 h at 80 ◦C. After filtering, the powders were washed with largemounts of water and dried at 110 ◦C. The resulting catalysts areenoted as M/K–Y type II to distinguish the metal-loaded zeoliteatalysts prepared in this study from those prepared in Ref. [13]. Foromparison purposes, K+ exchanged Y zeolites (K–Y) were preparedy the ion-exchange of 6 g Na–Y with 100 ml 0.8 M K3PO4 solutiont 80 ◦C that was stirred for 3 h. All as-prepared catalysts were cal-ined in a He flow at 615 ◦C for 4 h before catalyst characterizationnd/or catalytic reaction testing.

.2. Catalyst characterization

The chemical composition of the as-prepared metal-loadedeolites was determined by inductively coupled plasma atomicmission spectroscopy (ICP-AES, Varian Inc.) using an HF/HNO3olution to dissolve the zeolite samples. X-ray powder diffractionXRD) patterns were recorded at room temperature on a Rigakuiffractometer (Cu-K� radiation, Ni filter, 40 kV, 20 mA, 2� = 10–70◦,.05◦ step size, and 1 s count time). Temperature-programmededuction experiments with H2 (H2-TPR) were performed with

computer controlled flow reactor system (AMI-200, Altamira)quipped with a thermal conductivity detector (TCD). 200 mg ofatalyst was placed in a U-tube reactor and exposed to a 30 ml/minow of 10% H2 in Ar while the temperature was increased linearlyt a rate of 10 K/min from 50 to 700 ◦C. The sampling rate of theCD signal was 1 point every 10 s.

.3. Catalytic reaction tests

The ODH reaction of ethane was typically carried out using00 mg of catalyst in a tubular quartz micro-reactor operatingt atmospheric pressure. The weight of catalyst was varied inrder to obtain different C2H6 conversions at a fixed temperatureith a fixed feed flow rate. The reaction feed typically contained

eneral 381 (2010) 114–120 115

10.3 ml/min C2H6 (Air gas, purity >99.99%), 10.3 ml/min O2 (Air gas,purity >99.98%) and 68.2 ml/min He (Airgas, >99.999%) at 22 ◦C. Theeffect of the ratio of O2 to C2H6 in the flow on the yield of and selec-tivity for C2H4 was examined by varying the O2 flow rate from 5.8to 10.3 ml/min at a constant ethane and He flow rate. The reactionproducts were analyzed with a HP 5890 series II gas chromatographequipped with a TCD. Different columns were used to separate CO2,C2H4 and C2H6 (Porapak Q 80/100) and O2 and CO (molecular sieve5A). The was no observable conversion of ethane at 550 ◦C in eithera homogenous gas phase reaction or a reaction over quartz, andthere was less than 0.5% conversion of ethane at 600 ◦C for thesereactions. The conversion of C2H6 (XC2H6 ) and the selectivity (SC2H4 )of C2H4 and carbon oxides (SCOx , COx = CO or CO2) are based on thecarbon mass balance, which was between 95 and 100%, and wascalculated using the following equations:

XC2H6 = [CO] + [CO2] + 2[C2H4]2[C2H6]reactant

× 100 %,

SC2H4 = 2[C2H4][CO] + [CO2] + 2[C2H4]

× 100 %, and

SCOx = [COx][CO] + [CO2] + 2[C2H4]

× 100 %.

3. Results and discussions

3.1. Catalyst characterization

Table 1 lists the chemical compositions of the M–Y and M/K–Ytype II zeolites determined from ICP-AES. Based on the M/Aland Na/Al values the transition metal ions replace ∼2/3 of theexchanged ions in the original Na–Y zeolite. However, the metalloading (i.e. M/Al ratio) does not change (for example, Ni/Al = 0.36for Ni/K–Y type II and Ni–Y) after treatment using K3PO4 solutionsas evidenced by the fact that ([K]+[Na])/[Al] is ∼1.0 for all the M/K–Ytype II zeolites. These data show that effectively all the transitionmetal ions in M–Y zeolites were replaced by the alkali ions afterthe treatment with K3PO4 solution. The replaced M ions remainedon the zeolitic support as transition metal oxides or hydroxides.Small amounts of P were detected in the M/K–Y type II samples asindicated in the last column of Table 1. The XRD data for the tran-sition metal-loaded Y zeolites, in Fig. 1, show that the as-preparedM/K–Y type II and Ni–Y zeolites exhibit diffraction patterns similar

Co–Y 0.34 0.33Cu–Y 0.37 0.33Fe–Y 0.37 0.30Na–Y 0.83K–Y 0.06 0.96

116 X. Lin et al. / Applied Catalysis A: G

titHp

order of activity for these four ODHE catalysts is: Ni > Co > Cu > Fe,

Fig. 1. X-ray diffraction patterns of metal-loaded Y zeolites.

ype II relative to the untreated metal-loaded zeolites (M–Y). For

nstance, for Co–Y, no obvious H2-uptake peak was observed inhe TPR data (Fig. 2b, dotted line), whereas for Co/K–Y type II, a2 reduction peak starts at ∼550 ◦C which extends to higher tem-erature with a maximum at T > 700 ◦C. An H2-uptake signal was

Fig. 2. The H2-TPR profiles of 200 mg of M/K–Y type II zeolite (solid line

eneral 381 (2010) 114–120

observed starting at ∼350 ◦C for both Ni–Y and Ni/K–Y type II zeo-lites. However, above ∼425 ◦C the shape of the H2-uptake curvediffers markedly for the Ni/K–Y type II and Ni–Y zeolites. For thecases of Cu and Fe, treatment with K3PO4 solution leads to H2-reduction taking place at a lower temperature for M/K–Y type IIrelative to the untreated metal-loaded zeolites (M–Y). A plausibleexplanation for these differences in reductive behavior relates tothe bond energies for the metal–oxygen bonds for the differentmetals and different synthetic methods. Thus, the structures and/ormorphologies of these two sets of zeolites are different.

3.2. ODHE reactions on M/K–Y type II catalysts

3.2.1. Effect of the metalTable 2 presents the catalytic test results for the ODHE reactions

at 600 ◦C. All data in Table 2 were taken using the same O2 and C2H6flow rates and are the result of averaging GC data for two runs with atypical difference between runs of less than 2%. Only C2H4, CO, CO2,and a very small amount of CH4 were observed as reaction products.Because Na–Y and K–Y do not show significant catalytic activity forthe ODHE reaction under these conditions, the catalytic conversionobserved for the M/K–Y type II catalysts is a consequence of themetal loaded on these supports. From Table 2 it can be seen that the

which is somewhat different than the order reported for the bulktransition metal oxides (Co > Ni > Fe) [9]. These differences betweenthe unsupported and supported metal oxides could be a result ofsupport effects and/or differences in shape and/or morphology of

s) and M–Y (dot lines), where M = (a) Ni, (b) Co, (c) Cu and (d) Fe.

X. Lin et al. / Applied Catalysis A: General 381 (2010) 114–120 117

Table 2Reaction test result of ODHE on 500 mg M/K–Y type II catalysts listed in the Table.T = 600 ◦C. Flow: 10.3 ml C2H6 + 10.3 ml/min O2 + 68.2 ml/min He.

Catalyst Conversionof C2H6 (%)

Selectivity (%)

SC2H4 SCO SCO2

Ni/K–Y type II 18.8 77.9 3.4 18.7Ni–Y 9.4 77.7 11.6 10.7Ni/KYa 13.8 72.9 7.6 19.5Co/K–Y type II 17.3 30.7 5.0 64.3Cu/K–Y type II 17.0 23.6 1.8 74.6Fe/K–Y type II 11.7 35.5 7.8 56.7Na–Y 0.9 71.0 11.0 18.0K–Y 1.2 74.6 12.8 12.6

a Data were taken from [13]. Weight of catalyst: 100 mg; T = 600 ◦C; Flow: 9.7 mlC2H6 + 3.3 ml/min O2 + 62.2 ml/min He.

thI

orltveg

3

fevitti

conversion and selectivity

FC

Scheme 1.

he metal/metal oxide particles. The selectivity for C2H4 was theighest on Ni/K–Y type II at ∼78%, and the lowest on Cu/K–Y type

I at ∼24%.Ni–Y is less active for ODHE than Ni/K–Y type II (first two rows

f Table 2). This may be partly due to Ni2+ in Ni–Y being harder toeduce than in the Ni/K–Y type II zeolite, which is suggested by thearge difference in their H2-TPR profiles (Fig. 2a) in the 425–700 ◦Cemperature range. Compared to the Ni/K–Y catalyst reported pre-iously [13], where the Ni component was anchored as a result ofxposure to H2, the Ni/K–Y type II catalysts reported in this paperives a higher selectivity for ethylene at a higher C2H6 conversion.

.2.2. Effects of temperatureAccording to the generally accepted reaction scheme (Scheme 1)

or ODHE reactions the initial selectivity (at low conversion ofthane) to C2H4 depends on the k1/k2 ratio, while at higher con-ersions of ethane, the observed selectivity for production of C2H4

s associated with the ratio k1/(k2 + k3) [16,17]. In the following sec-ions the mechanistic implications of our data will be discussed inhe context of Scheme 1 above, and Scheme 2, which is discussedn Section 3.3.

ig. 3. A comparison of the conversion (a) and the selectivity towards C2H4 (b) for OD2H6 + 10.3 ml/min O2 + 68.2 ml/min He.

Scheme 2. Schematic diagram of a mechanism for catalytic ODHE reactions.

Fig. 3 shows the conversion and selectivity for the ODHE reac-tions at 550 and 600 ◦C for each metal-loaded zeolite. For allsystems, both the conversion of C2H6 and the observed selectivityfor C2H4 are greater at 600 ◦C than at 550 ◦C. That higher tempera-ture favors increased C2H4 selectivity suggests that these catalystshave similar ODHE reaction mechanisms. Thus for all the M/K–Ytype II catalysts examined in this work, the major primary productof oxidation of ethane is C2H4, since the selectivity and thus theratio k1/(k2 + k3) increases with temperature.

Fig. 4 shows the temperature dependence for C2H6 conversionand C2H4 selectivity over 500 mg of Ni/K–Y type II catalyst. TheC2H4 selectivity remains at ∼80% as the conversion increases up toover 23% as the temperature is increased from 500 to 620 ◦C at con-stant flow rate. The activation energy for a chemical reaction can beobtained by plotting the log of the rate of reaction versus 1/T. How-ever, unless the reaction is elementary this activation energy shouldbe viewed as a phenomenological activation energy that character-izes the temperature dependence of the overall mechanism (or therate limiting step) for conversion of reactant to product. The datain Fig. 5 allow for the calculation of the phenomenological acti-vation energy for the ODHE reaction on Ni/K–Y type II catalyst of51.5 ± 0.7 kJ/mol.

3.2.3. Effect of the ratio of O2 and C2H6 flow rates (FO2 /FC2H6 ) on

For all M/K–Y type II catalysts investigated in this study, theincreasing FO2 /FC2H6 increases the C2H6 conversion and lowers theC2H4 selectivity (see Fig. 6). The decrease in selectivity is relativelysmall for Ni (9.9%), but significant for Co (33.7%), Cu (25.4%) and

HE reactions at 550 and 600 ◦C catalyzed by 500 mg M/K–Y type II. Flow: 10.3 ml

118 X. Lin et al. / Applied Catalysis A: General 381 (2010) 114–120

Fig. 4. Temperature dependence of the C2H6 conversion and the C2H4 selectivity forODHE reactions catalyzed by 500 mg Ni/K–Y type II catalyst. The flow was: 10.3 mlC2H6 + 10.3 ml/min O2 + 68.2 ml/min He.

Fig. 5. A plot of ln(conversion) of ethane versus 1/T yields the apparent energyof activation for the Ni/K–Y type II catalyzed ODHE reaction. Flow: 10.3 mlC2H6 + 10.3 ml/min O2 + 68.2 ml/min He.

Fig. 6. The effect of FO2 /FC2H6 (the number in the figure correspond to the ratio FO2 /FC2H6

of catalyst: 500 mg. Temperature: 600 ◦C.

Fig. 7. FO2 /FC2H6 dependence of the C2H6 conversion and C2H4 selectivity of theODHE reaction on 500 mg Ni/K–Y type II at 600 ◦C.

Fe (25.6%). These data suggest that over-oxidation of the desiredproduct (C2H4) may be relatively more facile for Co, Cu and Feloaded zeolites than for Ni loaded K–Y zeolites. The k3/k1 ratioincreases more rapidly as a function of O2 concentration for Co,Cu and Fe than for Ni. To further characterize the slow decrease inC2H4 selectivity with increasing FO2 /FC2H6 ratio, more data points,were obtained and are shown in Fig. 7. Due to their mitigating effecton the production of carbonaceous deposits, which can lead to cat-alyst deactivation, O2 rich flows are desirable during ODH catalysisbecause they would be expected to enhance catalyst durability.However, an increase in relative concentration of O2 can lead tomore over-oxidation and thus less selectivity for ethylene produc-tion.

3.3. The C2H4 selectivity at zero conversion (S0) for Ni/K–Y type IIand their mechanistic implications

From Scheme 1 it can be seen that at very low conversions ofethane the selectivity depends on the ratio k /k . Thus, the k /k

1 2 1 2ratio can be estimated from the selectivity in the limit of zero con-version, which we will designate as S0, where S0 is obtained byextrapolating the ethane conversion to zero as discussed in Section3.2.2.

) on the C2H6 conversion (left) and C2H4 selectivity for the ODHE reactions. Weight

X. Lin et al. / Applied Catalysis A: General 381 (2010) 114–120 119

Fig. 8. Variation of the C2H4 selectivity as a function of conversion on Ni/K–Ytype II at 550 ◦C (closed rectangles) and 600 ◦C (open rectangles). Flow: 10.3 mlC2H6 + 10.3 ml/min O2 + 68.2 ml/min He. S0(550) and S0(600) are obtained from theet

tstttesaarTb

cCFnoctg5it(isoimtaodiocid

Fig. 9. Variation of the C2H4 selectivity for the ODHE reaction as a function of C2H6

conversion on the Ni/K–Y type II catalyst at 600 ◦C for different O2/C2H6 concentra-

of activation for the reaction of ethane on Ni/K–Y type II is

xtrapolated values for the C2H4 selectivity at zero C2H6 conversion at the respectiveemperatures.

Kung [18] suggests that the first step for ODH reactions is activa-ion of a C–H bond to form a metal alkyl or metal alkoxide. For thepecific case of transition metal oxides, our previous work [13] onhe Ni/H–Y catalyzed ODHE reaction allowed us to conclude thathe mechanism depicted in Scheme 2, in which the first step ofhe ODHE reaction is to activate C–H bond in C2H6 to form a metalthoxide, is consistent with our data. The metal ethoxide could sub-equently undergo �-H abstraction to produce an aldehyde or �-Hbstraction to produce ethylene. In Scheme 2 k�/k� is function-lly equivalent to k1/k2, although k1 is not identical to k� since theate determining step involves conversion of C2H6 to the ethoxide.herefore, the C2H4 selectivity at zero conversion (S0) is controlledy k�/k� and this ratio can be estimated from S0.

We now focus on the Ni/K–Y type II catalyst. S0 for Ni/K–Y type IIatalyzed ODHE can be estimated by extrapolation of the curve for2H4 selectivity versus C2H6 conversion (see Figs. 8 and 9). Fromig. 8, S0 at 550 ◦C is ∼87%, which increase to ∼91% at 600 ◦C. It isot clear the this difference in S0 is greater than the error limitsn S0, given that we have a limited number of data points and theurves are generated from a spline fit. However, we have consis-ently observed an increase in both the yield and selectivity for aiven quantity of catalyst when the temperature is increased from50 to 600 ◦C. This is compatible with an increase in S0. This behav-

or indicates that in the context of the model in Scheme 2, increasedemperature favors �-H abstraction over �-H abstraction [i.e. k�/k�

550) < k�/k� (600)]. Fig. 9 demonstrates that for all O2/C2H6 ratiosnvestigated the S0 values were between ∼91 and ∼96% (this corre-ponds to k�/k� in the range of ∼10–20). That is, at the temperaturef 600 ◦C, over 90% of C2H6 molecules are converted to C2H4 evenn a relatively O2 rich flow (“O2 rich” is with respect to the stoichio-

etric equation for ODHE: C2H6 + 1/2O2 = C2H4 + H2O). We notehat the O2 consumption never reached 100% and though smallmounts of CH4 were observed above 600 ◦C, hydrogen was notbserved, indicating that there was minimal non-oxidative dehy-rogenation. Additionally, no acetic acid or acetone was observed

n the reaction products. However, a small amount of coke wasbserved on the catalyst after catalytic testing. As such, a CO2 peak

ould be observed when this catalyst was exposed to a flow of O2n He. This result indicates that a small amount of non-oxidativeehydrogenation processes does take place above 600 ◦C.

tion ratios in the reactant flow: (A, rectangles and solid line) 1.0, (B, cycles anddot line) 0.84, (C, triangles and dash line) 0.71 and (D, stars and dash dot line)0.56. SA

0 , SB0 , SC

0 , and SD0 are the extrapolated values for C2H4 selectivity at a C2H6

conversion of 0%.

The observed change in selectivity and yield of the two sets ofcatalysts, prepared via different methods (see Section 3.1.1), wouldtypically be rationalized as due to a change in size, composition,and/or morphology of the catalyst particles. However, the fact thatthe selectivity at low S0 is associated with the ratio k�/k� indicatesthat the relative magnitudes of these rate constants change for thedifferent catalysts. Thus, the changes in selectivity cannot be dueto simply a surface area resulting from a change in particle size:There must be either a change in particle morphology, most likelyleading to preferential exposure of different crystal planes, and/ora change in the steady state proportion of oxide and/or metal.

With the advances in density functional theory (DFT) it hasbecome feasible to attempt to calculate structures and other param-eters for reactions on surfaces, including oxide surfaces [19].Calculations of ka and kb for metal-loaded zeolites are a potentialchallenge for such DFT methods. The data obtained in this studycould provide a calibration for such calculations, whereas the cal-culations could provide additional insights into the details of themechanism for ODHE and why specific metal oxide catalysts exhibithigher yields and selectivities than others.

4. Conclusion

(1) Four first-row transition metal (Fe, Co, Ni and Cu) oxide loadedK–Y zeolites were synthesized and characterized, and theircatalytic performance for ODHE was examined. At 600 ◦C, forequal concentration of O2 and C2H6, the ODHE activity fol-lows the order: Ni > Co > Cu > Fe and the C2H4 selectivity is:Ni > Co ≈ Fe > Cu. The Ni/K–Y type II catalyst can give a higherselectivity for ethylene at higher C2H6 conversions than pre-viously reported for a Ni/K–Y catalyst prepared via a differentmethod.

(2) Increasing reaction temperature leads to a modest increase inselectivity for all M/K–Y type II systems. The apparent energy

51.5 ± 0.7 kJ/mol.(3) Increasing the concentration of O2 (FO2) relative to C2H6 (FC2H6 )

in the reactant flow leads to an increase in the conversion of

1 is A: G

(

A

aoaPp(U

[[[

[

20 X. Lin et al. / Applied Catalys

C2H6, but a decrease in the C2H4 selectivity. The influence ofFO2 /FC2H6 on the C2H4 selectivity is relatively small for Ni/K–Ytype II, but more significant for the other M/K–Y type II zeolites.

4) The C2H4 selectivity at zero conversion (S0) on Ni/K–Y type IIwas obtained by extrapolation of the yield to zero. The val-ues for S0 are consistent with the rate of �-H abstractionbeing at least 10 times that of the rate of �-H abstractionfrom a metal ethoxide formed from ethane via an ethyl radicalintermediate.

cknowledgments

This work was supported by the Chemical Sciences, Geosciencesnd Biosciences Division, Office of Basic Energy Sciences, Officef Science, U.S. Department of Energy (DE-FG02-03-ER15457),

t the Northwestern University Institute for Catalysis in Energyrocesses. This work made use of the J.B. Cohen X-ray Facility sup-orted by the MRSEC program of the National Science FoundationDMR-0520513) at the Materials Research Center of Northwesternniversity.

[[[[[[

eneral 381 (2010) 114–120

References

[1] Chemical & Engineering News 84 (2006) 59.[2] K. Weissermel, H.-J. Arpe, Industrial Organic Chemistry (C.R. Lindley, Trans.)

3rd completely rev. ed., VCH, Weinheim, 1997, p. 59 (Chapter 3).[3] F. Cavani, N. Ballarini, A. Cericola, Catal. Today 127 (2007) 113.[4] F. Cavani, F. Trifirò, Catal. Today 51 (1999) 561.[5] L. Capek, J. Adam, T. Grygar, R. Bulánek, L. Vradman, G. Kosová-Kucerová, P.

Cicmanec, P. Knotek, Appl. Catal. A 342 (2008) 99.[6] T. Blasco, A. Galli, J.M. Lopez Nieto, F. Trifirò, J. Catal. 169 (1997) 203.[7] G. Tsilomelekis, A. Christodoulakis, S. Boghosian, Catal. Today 127 (2007) 139.[8] C. Liu, U.S. Ozkan, J. Mol. Catal. 220 (2004) 53.[9] Y. Schuurman, V. Ducarme, T. Chen, W. Li, C. Mirodatos, G.A. Martin, Appl. Catal.

A 163 (1997) 227.10] X. Zhang, J. Liu, Y. Jing, Y. Xie, Appl. Catal. A 240 (2003) 143.11] K.-I. Nakamura, T. Miyake, T. Konishi, T. Suzuki, J. Mol. Catal. A 260 (2006) 144.12] P. Concepcion, T. Blasco, J.M. Lopez Nieto, A. Vidal-Moya, A. Martınez-Arias,

Micropor. Mesopor. Mater. 67 (2004) 215.13] X.-F. Lin, C.A. Hoel, W.M.H. Sachtler, K.R. Poeppelmeier, E. Weitz, J. Catal. 265

(2009) 54.

14] Q. Sun, W.M.H. Sachtler, Appl. Catal. B 42 (2003) 393.15] B.-Z Zhan, E. Ignesia, Angew. Chem. Int. Ed. 46 (2007) 3697.16] J.M. Blasco, Löpez Nieto, Appl. Catal. A 157 (1997) 117.17] M.D. Argyle, K. Chen, A.T. Bell, E. Iglesia, J. Phys. Chem. B 106 (2002) 5421.18] H.H. Kung, Adv. Catal. 40 (1994) 1.19] M.S. Palmer, M. Neurock, M.M. Olken, J. Am. Chem. Soc. 124 (2002) 8452.